Windows 10 cannot reach 127.0.0.1

On my Windows 10 PC, no browser or app can reach 127.0.0.1. This is true even with the Firewall turned off.
Firefox says "Unable to Connect"
Plex says "Timeout connecting to server 127.0.0.1"
Edge says "Hmm, we can't reach this page"
This is a new PC, so as far as I know it has never been possible to do this.